The characteristics of synapses developing in vivo will be studied by electrophysiological and electron microscopic techniques. Several different preparations will be utilized. (1) The regenerating limb of the adult salamander is a source of synapses developing on muscle fibers which have not been previously innervated. Examination of this system has already provided information about the development of normal synaptic function, and this project will be continued. (2) Parasympathetic neurons located on the interatrial septum of the frog heart form a simple pattern of normal innervation and are quite accessible for physiological study. These cells are denervated by bilateral vagotomy. The features of synapse development during the course of reinnervation of these ganglion cells will be studied.